Conversion system



June 9, 1942- J. VON ISSENDORFF EIAL 2,235,634

CONVERSION SYSTEM Filed Dec. 19, 1939 ATTORNEY Patented June 9, v1942CONVERSION SYSTEM Jiirgen yon Issendorfl and Karl Maertens, Berlin-Siemensstadt, Germany, alllgnorl to Wettinghouse Electric 8:Manufacturing Company, East Pittsburgh, Pa., a corporation ofPennsylvania Application December 19, 1939, Serial No. 310,006

Germany December '10, 1938 9 Claims. (Cl. 175-363) Our invention relatesto a conversion system and particularly to a protective device for sucha system.

For rectifier plants of substantial size with two or more parallelunits, different protective circuits have been proposed to render theeffect of backfire as far as possible harmless. In particular, highspeed circuit breakers in the D. C. circuit or high speed relays in thegrid circuits have been used for interrupting the short circuit currentswhich arise. For example, an arrangement has been provided in which allof the rectifiers are blocked for a certain time in case of adisturbance by means of the grids; however, the rectifiers which are notdefective are not blocked so long as those in which the backfire istaking place. Here there is only one sequence which may be maintained bya switchboard attendant by hand actuation without special considerationbecause he always carries out control measurements at the faultyrectifier before reclosure.

The reactions on the feeding network are of particular significance fora high current plant with many parallel operating rectifiers; since therectifier loading often represents a considerable portion of the totalloading of this network. Abrupt variations or even completedisconnection of the whole load will always have a harmful effect on themaintenance of potential as well as on the stability of the feedinggenerators.

In accordance with ourinvention, the blocking potential, which isusually automatically applied to the grids of all rectifiers which aredrawn into sympathetic operation in the event of an areback, ismaintained connected in only for a time as long as the disconnectingtime for the overcurrent in the faulty rectifier. After the extinctionof the short circuit current loading of the network, the normalutilization load at approximately full value is accordinglyre-established at once. Thereby a complete unloading of the network isavoided or is limited to a fraction of a period. It is advantageous toset the time sequence of the mechanical or ionic relays necessary forthis purpose in such manner that the load fluctuations in the polyphasenetwork and the phenomena associated therewith such as the blinking ofthe lights of the lamps connected to the circuit become a minimum.

The carrying out of these measures is not dependent alone on the gridcircuits, but there are interconnections with circuit arrangements onthe D. C. side as well as on the polyphase side of the rectifier plantwhich are applicable with adgrids may be made dependent on the positionof a back current switch in the D."C. circuit and an A. C. switch in thefeeding conductor. The short circuit power is considerably decreased atthe time that the D. C. back current switch drops out; it is at thelatest interrupted finally by the A. C. switch. Therefore, it is atleast possible, by using front and back contacts on these switches, toestablish the characteristics of the plant and the extinction propertiesof the rectifier in a manner corresponding to the release of the grid sothat there is a minimum of load fluctuation. Corresponding operation isalso attainable with the aid of additional relays which are influencedby the over-current or a potential which changes in the case of a faultand raise the blocking only at the interruption of this infiuence. Oftenit will be preferable to time the release of the grid with adjustabledelay devices of mechanical or electrical type (reservoir means) in suchmanner that the unfaulty rectifiers do not deliver current before theseparation of the faulty rectifiers from the D. C. circuit, but alwaysbefore the current flowing between the anodes of the faulty rectifiersis completely interrupted. In this manner, the current fluctuations ofthe feeding network are decreased to a minimum.

The rectifier in which the backfire .occurs need not be unconditionallyautomatically connected in again to fulfill the problem with which weare confronted; this may also take place by hand, if. for example, thevacuum control is carried out. Therefore, a locking of the faultyrectifier is sufficient sometimes in dependence upon cathode backcurrent which prevents the release of the grids or the connecting inagain of the associated circuit interrupters.

It is, accordingly, an object of our invention to provide a system inwhich faults will be interrupted with a minimum of reaction on thefeeding network.

It is a further object of our invention to provide a multiple unitconversion system in which a faulty unit is isolated and the non-faultyunits assume load before interruption of the faulty current.

Other objects and advantages will be apparent from the followingspecification taken in conjunction with the accompanying drawing, inwhich I Figure 1 is a schematic illustration of a conversion systemaccording to our invention, and

Fig. 2 is a graphical illustration of the current vantage. For example,the unblocking of the conditions in our converter system.

In Fig. 1, an exemplary embodiment of the invention is illustrated. Thetransformers 2 are connected to the'polyphase network I through powercircuit breakers I. The secondary phase potentials are supplied to therectifiers 3. In the D. C. circuit of the latter, there are in. seriesthe shunt impedance 4, the back current switches ii, l2, ii, the D. C.network 2 and the smoothing chokes I. lel which may be selected at anynumber are here elected to be 3. The quick acting relays 8, and I andthe batteries 2| serve for impressing on the rectifier grids a negativeblocking potential with reference to the-cathode. The release of therelays 8It is effected by the windings l2 connected to the shunt 4. Thewindings II which may be excited from the battery 26 serve for return ofthe relays, that is to release the grids. The back current isinterrupted by the cooperation of relays I3 and I4.

The operation of the protective device as a consequence of backfire isas followsrAssume an arc-back to occur in any section such, for example,on the left-hand section of Fig. 1. Then the ure-back in the left-handsection produces a short circuit on the D. C. line 6 so that all theremaining sections are momentarily overloaded. The over current in theimpedances 4 energizes the coils l2 so that the quick acting relays 6,Q, and I0 pick up as shown in Fig. 1. Simultaneously the reverse currentswitch l5 drops out removing the iaulty section and clearing the shortcircuit on the line 6. The reverse current switch I! on opening closesits back contacts Zl and25. Closing the back contact 24 connects theconductor It with the conductor is. The reversing conductor I9 isconnected through the relay l3 and the front contact 26 of the relay Hwith, the battery 30, the other terminal of which is connected to thereversing conductor 20. The two reversing conductors i8 and now form aD. C. conducting bar which supplies current for returning the quickrelays by means of the winding H, assuming that the front contacts 23 inseries with them are closed. In Fig. 1, the operating contacts 23 areonly closed for the switches l6 and I1 so that, accordingly, only thequick acting relays 9 and iii of the unfaulty rectifiers are returnedand these again take up their normal current supply to the collectingbars 6.

The return current also actuates the relay l3,

' which, under certain circumstances, with a small delay, closes itsfront contacts 28 after the return process of the quick relays 8-! hasbeen completed. Thereby the winding of relay [4 which heretofore wasexcited through the resister 29 from the battery 30 is short circuited.The relay I4 consequently drops out and closes its back contacts 21which are also connected in parallel to its windings albeit with thereversing conductor 2| connected in series. Finally, these two reversingconductors are connected to' each other through the back contact of thedropped out switch I5, thus the relay i4 is held in its rest position.Its interrupted front contacts 26 prevent another return, i. e., in caseof another backfire arising for the rectifiers which have been connectedin again whereby the quick acting relays would operate a second time,the whole plant would remain blocked.

The operator need now concern himself only with the switch l5 and thequick relay 8 of the backflring rectifier. After a 'sufiicient timeinterval, the switch I5 is again closed opening back contacts 24 and 25and the quick relay 8 is re- The rectifiers operating in paralon thefalling 'out of a part of the rectifier beaasacsa i The normal operatingcondition is, in this way.

again established; for by the interruption of the back contacts 25 onswitch II, the connection of the reversing conductors 2| and 22 islifted and the short circuiting of the winding of relay !4 is suppressedso that it pulls up and by closing its front contact 2' brings thereturn circuit again into operating condition. Finally, by closing theswitch IS, the back contact 24 which has initiated the operation of theabove-described protective device is interrupted.

After the dropping out of the switch 15, the backfire of the associatedrectifier 3 is immediately extinguished by grid-blocking, since the backcurrent from the bus bar 6 is interrupted. Therefore, the feedingnetwork I is again loaded without interruption by the immediate returnof the quick relays! and II. If it happens, however, that as aconsequence of high temperature of the anodes, the backfire extends to aplurality of anodes, and is not extinguished, then the associated powercircuit breaker i must operate. In accordance with the invention, acorresponding backcontact of the power circuit breaker switch may also,for such a case, be selected in lieu of the back contacts 24 of thequick switches l5, l6 and IT for the connection of the reversingconductors i8 and It, or a series circuit of the hack contacts of theswitches which are associated with one and the same rectifier may beused. Finally, intermediate relays with adjustable delay may also beprovided and they may effect the return of the quick relays after a timeselected to correspond to the extinguishing of the backfires by theswitches present. vFinally, we may start with the instantaneous value ofthe current in the faulty rectifier and in such a case, the return ofthe quick relay takes placeonly when the faulty rectifier or itstransformer carries current under a definite limiting value. Withadvantage, ionic relays may be used in such a case.

With advantage, the automatic switching .in accordance with theinvention may be completed by regulating devices which maintain constanta definite potential or a definite current strength in the rectifier innormal operation, and after the falling out of one or more rectifiers,atonce effect a decrease; For example, the total current strength may beheld constant and after the falling away of the faulty rectifier, thewhole regulator may be influenced with the aid of relays in such mannerthat it controls down to a decreased current strength which is normalfor the rectifiers remaining in operation. This reaction may be obtainedby interrupting of shunts to the current coils of the regulator withoutfurther complications. It is also advantageous to control each rectifierby an individual regulator to constant current because in such a case,theseparate influencing of the regulation comes superfluous.

In Fig. 2, the course of the current strength as a function of time in aprimary phase of a plant protected in accordance with the invention isillustrated diagrammatically. Up until the time t1, the normal networkcurrent flows to the rectifier. It is assumed that at t1, an arc-backarises and the current rises considerably. At t:,

nized from this that in the power supply (steam consumption) of a powergenerator, no appreciable interruption is introduced.

We claim as our invention:

1. Protective device for'current rectifier plants, comprising aplurality of parallel operating mercury vapor rectifiers, grid controlmeans for each of said rectifiers, a source of blocking potential foreach of said control means, means responsive to are back in any of saidrectifiers for applying said blocking potentials to all of said controlmeans, an arc back responsive disconnecting means in series with each ofsaid rectifiers, and means responsive to the opening of thedisconnecting means of the rectifier undergoing are back for removingthe blocking potential from the control means of the remainingrectifiers.

2. Protective device according to claim 1, characterized by the factthat the grid blocking of all the rectifiers is so dimensioned asregards the length of time during which it is applied that a minimum ofload current fluctuation arises in the feeding alternating currentnetwork,

3. Protective device according to claim 1, characterized by the factthat the blocking of all of the rectifiers sets in instantaneously atthe beginning of the short circuit and is maintained until the rectifierwhich is backfiring is disconnected from the feeding network as well asfrom the supplied network.

is impressed on the control grids of all the parallel rectiflers withoutdelay as long as any of the rectifiers carries an overcurrent.

6. Protective device according to claim 1, characterized by the factthat the removal of the blocking potential is eflfected by auxiliarycontacts of the mechanical switch which disconnects the backfiringrectifiers.

7. Protective device according to claim 1, characterized by the factthat the repetition of a backfire before the first backfir'ing rectifieris again connected in looks the repeated release of the grid blockingand renders the blocking final.

8. An electric conversion system comprising a plurality of paralleloperating arc type converters, grid control devices for each of saidconverters, a high speed reverse current switching device in series witheach of said converters, an overcurrent relay in series with each ofsaid converters, a source of blocking potential for each of saidconverters, switching means actuated by said overcurrent relays forapplying said blocking potential to said grid control devices and meansresponsive to the opening of the reverse 4. Protective device accordingto claim 1, characterized by the fact that the blocking time isdetermined by influencing the grid control potential by means of a relaycombination or by electrical reservoir means of adjustablecharacteristic time. 7

, 5. Protective device according to claim 1, characterized by the factthat the blocking potential current switching device of the faultyconverter for removing the blocking potential from the non-faultyconverters.

9. An electric conversion system comprising a plurality of paralleloperating arc type converters, grid control devices for each of saidconverters, a high speed reverse current switching device in series witheach of said converters, an overcurrent relay in series with each ofsaid converters, a source of blocking potential for each of saidconverters, switching means actuated by said overcurrent relays forapplying said blocking potential to said grid control devices and meansresponsive to the opening of the reverse current switching device of thefaulty converter for removing the blocking potential from the non-faultyconverters and means responsive to a second fault for locking out theconversion system.

JIJRGEN you rssrmnom. KARL MAERTENS.

